As a general rule, a lower cost product reflects a smaller amount of resources used in making it. I'm reasonably certain that the production of a paper coffee cup uses fewer resources than the production of a ceramic coffee mug. So if I was going to purchase only 1 cup of coffee, it the environmentally friendly purchase would be the styrofoam cup.
But of course I'm not buying just one cup of coffee, I'm a daily coffee drinker. That means I would use many more styrofoam cups than ceramic cups over the course of a lifetime. The problem is that as a consumer of water and energy, I don't know the marginal cost of those resources as applied to the washing of a mug, and there are several variables involved, such as:
1. If I drink 3 cups of coffee a day, am I reusing the same paper cup, or getting a new one each time? If I'm buying it from the coffee stand in our student union, it will probably be a new one each time, but if I'm making coffee in our break room or at home, I'll use the same cup all day (or even two to three days, depending on how fastidious I am).
2. Am I washing the coffee mug after every use with hot water? Or am I washing it once a day with the rest of my dishes? In the latter case, the only extra energy/water used is the water used to rinse the soap away after cleaning.
3. How long do I actually keep a ceramic mug, on average? I'm better than most people at losing things. I've gone through at least 4 travel mugs in the last 4-5 years, which strengthens the case for disposable cups, but I've been using the same mug at home for close to a decade.
I can plot these on a simple continuum, to see the best and worst case scenarios.
Break ceramic mugs regularly.....................Keep the same mug for decades
Wash in diswasher after each use..............Wash only when doing other dishes
Reuse paper cups multiple times.................Toss paper cups when empty
On the left side of the continuum, I'm disposed to listen to the claim that I'd be a better friend to the environment by using paper cups. But on the right side, I'd be very very dubious that disposables are better than reusables. I'm somewhere in the middle-right. I actually do reuse paper cups regularly; for example when I buy my first cup of the day at the coffee stand, then get my other cups from the breakroom, I frequently reuse the paper cup from my earlier purchase, which would incline me to the left side. But, with the regrettable exception of travel mugs, I keep the same ceramic mug for years and rarely wash it except when doing other dishes (I'm not too fastidious about a dirty coffee mug) , which pushes me more towards the right side of the scale.
Overall, I'm inclined to think that I should probably continue using a ceramic mug. But I can't be sure, because while I can figure out those personal factors, I simply don't know how much it costs me to wash my coffee mug. I suspect it's just not that much, which satisfies me with my own economc choice, but doesn't satisfactorily answer the important question:
Addendum: I think I'll use this as an exercise in my environmental politics course next spring. My plan for the course is twofold: (1) to teach students the structure of environmental policymaking (primarily in the U.S., but, if time allows, some focus internationally as well); (2) to teach them to analyze environmental claims rather than just accept them based on their political predispositions.
Any suggestions for good readings, issues to include, exercises, etc., are welcome.